Thorium compound protective coatings for reflecting surfaces



P 1947. G. L. DIMMICK 2,427,592

THORIUN COMPOUND PROTECTIVE COATINGS FOR REFLECTING SURFACES Filed July 31, 1943 Wifness- Patented Sept. 16, 1947 THORIUM COMPOUND PROTECTIVE COAT- IN GS FOR REFLECTING SURFACES Glenn L. Dlmmick, Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application July 31, 1943, Serial No. 496,929

9 Claims.

1 My invention relates to light transparent protecting coatings for reflecting surfaces such as be selectively connected to an electrical heating source 23 through vacuum tight insulated leadfront surface mirrors used in optics, ordinarily ins 21 and 28 respectively, and a switch 25. Elemade by evaporating in vacuo aluminum or silment (9 is preferably a platinum or molybdenum ver upon a glass support base. boat containing thorium oxy-fiuoride powder 20, An object of my invention is to provide an imwhile 2| is a heating coil of tungsten adapted to proved protective coating on a front surface mirreceive a piece of aluminum 22. When silver is r ror that prevents oxidation and corrosion while employed, the wire should be of tantalum or having substantially no adverse effect upon the molybdenum. optical properties of the mirror. 10 After the desired degree of vacuum has been Another object of my invention is to provide attained, and the residual gas cleaned up by an improved thin transparent coating for promeans of an electrical discharge between magtecting a light reflecting metallic surface from nesium gettering electrodes 24 which also cleans oxidation and corrosion and which is highly the surfaces of the devices to be coated, the durable in regard to abrasion and certain maswitch 25 is operated to connect the heating terials such as water, salt spray and alkali vapors. source 23 to the boat l9 to heat the thorium com- Heretofore, numerous attempts have been made pound to a degree causing it to evaporate and to apply protective coatings to front surface mirdeposit upon the exposed surfac rors, but the materials and process used have re- 5 and 1. After a sufficient thickness of a prime suited either in a coating that adversely affected layer has been deposited, the heating source is the desirable optical properties or one that lacked disconnected from boat l9 and connected to heat the requirements of durability, chemical and the coil 2| containing an element 22 of aluminum mechanical. or silver to cause a layer of aluminum or silver In accordance with my invention, I have proto be deposited. After the desired thickness of videdathin durable protective coating forafront 25 the layer has been effected. the heating cursurface mirror of the aluminum or silver derent is switched back to element l9 to evaporate posited type by evaporating in vacuo a thin film more of the thorium compound and deposit a thorium oxy-fluoride upon the mirror surface. superficial protective layer of this material of the Reference is made to my application Ser. No. order of V4 Wave en h in thickness- 464,018, filed October 31, 1942, for a more com- By way of example of an actual application plete understanding of the nature of my novel of the foregoing process, a thin mirror surface coating material. was formed on the hypotenuse side of a prism For a more complete understanding of the inwhich is indicated at 5 in Fig. 1, and in Fig. 2. vention, reference may be made to the drawings One boatload of the ThOFz, probably enough for in which, three and one-half wavelengths thickness of Fig. 1 shows in front elevation a cross sectional layer, was evaporated onto the surface. Next view of apparatus, reduced in size, used in carrythe aluminum was applied until the evaporating ing out my invention, filament could just be seen through the coating. Fig. 2 is a greatly enlarged sectional view of a ThOFz was then applied on top of the aluminum front surface mirror embodying the invention, 4 layer. The prism was removed to an oven, and Fig. 3 is also an enlarged sectional view of a with others was baked at about 175 C. for forty front surface mirror embodying a modification of hours, thereby producing a hard durable coating. the invention, Heretofore it has been customary to bake coated Fig. 4 is a similar view of a front surface mirelements for a few hours without removing them ror embodying the invention, from the bell jar, but I have found that it is more Fig. 5 is a similar view of a front surface mirror desirable from a production standpoint to remove embodying a preferred form of the invention. such elements from the jar and bake them in an Referring to Fig. 1, a plurality of optical devices oven under properly controlled thermal condi- 3, 5 and I are mounted of suitable support structions. ture 9 and II within a bell jar l3 supported by In adapting the above process to production and sealed to a flat surface metal slab l5. A measured amounts of evaporating material, prefvacuum pump apparatus, not shown, is conerably in the form of pellets for easy handling, nected through a conduit H to an opening in the are placed in separate heating supports. The slab l5. Below the devices to be coated are aramount in each pellet is determined in accordance ranged heating elements is and 2| adapted to with the desired thickness of the layer to be deposited by evaporation of the pellet. A time controlled mechanism can be used for sequence operation of the switch 25.

In the above example, the exact thickness of either of the layers of thorium oxy-fluoride is not very critical, but the top and final layer should be at least one-quarter of a wavelength in thickness. The material that is applied directly to the glass does not necessarily have to be thorium oxy-fluoride, but could be zinc sulfide or bismuth fluoride. In such case, a thin layer of thorium oxy-fluoride can to advantage be applied first to the glass to obtain good adhesion before applying the zinc sulfide in that the latter adheres better to the thorium compound than to glass. The thorium compound adheres to glass under unfavorable conditions and seals the glass against water absorption. The aluminum layer is then applied on top of the zinc sulfide and a final layer of the thorium compound applied over the aluminum, as shown in Fig. 3.

I have found, however, that if the glass is good and clean it is not necessary to apply anything to the glass before applying the aluminum layer, and in Fig. 4 I have shown this simplified arrangement, wherein the aluminum layer is deposited directly on the glass base, followed by a top layer at least of the order of one-quarter wavelength of the desired light in thickness of the thorium compound. The latter layer prevents the usual oxidation of the aluminum in that it is applied while still in the vacuum chamber, over the aluminum layer.

In Fig. 5 I have shown the application of my invention to a front surface mirror using a silver layer. Silver films are known to have still better properties than aluminum, as a reflector, as to emciency, particularly for certain parts of the spectrum. Whereas the oxide protects aluminum films, silver has no such inherent protection and is readily tarnished, making it unsuitable as a front surface mirror when exposed to the atmosphere. I am aware that attempts have been made to overcome this difiiculty, reference being made to the highly meritorious text of Strong, Procedures in Experimental Physics, published by Prentice-Hall, 1939, pages 186 and 187. On page 187 of Strongs book he describes the method of protecting silver films with calcium fluoride (or quartz) and also with a thin film of aluminum which is allowed to oxidize and thus protect the silver with transparent aluminum oxide. Calcium fluoride is not a good material to use as a protection because it is quite fragile and is easily damaged by cleaning it in the normal way. Aluminum oxide is probably a much better material for protecting silver, but it is very difiicult to evaporate the oxide itself and if one follows the procedure outlined by Strong, it is' only possible to provide a few molecules of aluminum oxide because if a thicker layer of aluminum is deposited, the outer surface will oxidize and prevent oxidation to any appreciable depth. Thorium oxy-fiuoride may be evaporated in any thickness desired, is easy to apply and is very effective in its protecting action.

While I prefer to apply my protective coating in vacuum immediately following the deposit of the reflecting material, it can be applied to a mirror which has previously been formed by some process other than the vacuum evaporating method.

The ThOF: is prepared by heating thorium fluoride ThF'rHzO in a platinum crucible in a vacuum or in an inert gas. When the thorium While there is reason to believe that the above compound ThOFz is formed, it is possible that the water of crystallization is merely driven off, leaving the ThF4, and I do not wish to be restricted to thorium oxy-fluoride when the resulting compound may be thorium fluoride.

The compound comprising thorium and fluorine prepared in this manner is placed in a platmum boat and heated to about 1000 degrees C. in a vacuum or inert atmosphere. I prefer to use the term inert atmosphere to include a vacuum or an inert gas. At this temperature the compound melts and evaporates and it may be evaporated onto the optical surface to an appropriate thickness. The evaporated film of this material after it has been baked for several hours at a temperature of 70 C. or somewhat higher, has a number of advantages over other types of films in that it repels water, it is extremely hard, it has a very low surface friction, it is not soluble in pure water, salt water, or in alkaline solutions, it forms a very good bond with the glass and it is clear and transparent without appreciable absorption in the visible spectrum.

It is obviously unusual to provide a film of such extreme uniform thinness, about flve millionths of an inch, that affords such a high degree of protection, while having no undesired optical effects. Protective coatings of lacquer or other water resistant coatings are of course out of the question for precision optical devices because of non-uniformity in thickness and uneven surface, undesired light or color effects, etc. The coating is so efiective that when applied to the best optical glass that is too hygroscopic for practical purposes, it seals it against the deleterious effects of water. A sample lens of such material was coated over half its surface with my thorium compound according to my invention, and subjected to a water vapor test. The coated part emerged in perfect condition, while the uncoated part was all corroded and unsuitable for optical purposes. It will be understood that other modifications are possible without departing from the spirit and scope of my invention. My invention, therefore, should not be restricted except in so far as it is necessitated by the prior art and the spirit of the appended claims.

I claim as my invention:

1. In combination, a glass base, a relatively thin layer of an adherence material deposited on said base, a specular reflecting layer deposited upon said layer of adherence material, and a transparent layer of a protecting material deposited upon said reflecting layer, said adherence material and said protecting material being composed of a compound comprising at least thorium and fluorine formed by heating ThF4-H2O at red heat in an inert atmosphere.

2. In combination, a front surface mirror and a superposed transparent layer of a compound comprising at least thorium and fluorine for protecting said mirror against corrosion and abrasion, formed by heating monohydrated thorium tetrafluoride at red heat in an intert atmosphere.

3. In a, light-reflecting device, a support base. a layer of high reflecting material on said base forming a specular reflecting surface, and a superposed transparent layer of a compound comprising at least thorium and fluorine, formed by heating monohydrated thorium tetrafluoride at red heat in an inert atmosphere. 4. In combination, a front surface specular light reflector and a. transparent layer of a compound comprising at least thorium and fluorine deposited thereon of a thickness at least of the order of one-quarter of a wavelength of the light to be reflected, formed by heating monohydrated thorium tetrafluoride at red heat in an inert atmosphere.

5. In combination, a front surface specular light reflector of a material that is readily affected deleteriously by the atmosphere, and a transparent layer of a compound comprising at least thorium and fluorine over the surface of said reflector, formed by heating monohydrated thorium tetrafluoride at red heat in an inert atmosphere.

6. In combination, a front surface silver mirror and a superposed transparent layer of a compound comprising at least thorium and fluorine for protecting said mirror against corrosion and abrasion, formed by heating monohydrated thorium tetrafluoride at red heat in an inert atmosphere.

7. In combination, a front surface mirror of aluminum, and a superposed transparent layer of a compound comprising at least thorium and .fluorine of a thickness at least of the order of one-quarter wavelength of the light to be reflected, formed by heating monohydrated thorium tetrafluoride at red heat in an inert atmosphere.

8. A front surface mirror, comprising a vitre- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,301,456 Sabine Nov. 10, 1942 2,084,327 Edwards June 22, 1937 2,179,491 Cain Nov. 14, 1939 2,079,784 Williams May 11, 1937 2,237,123 Teves Apr. 1, 1941 2,207,656 Cartwright July 9, 1940 1,675,120 Marden et a1 June 26, 1928 2,366,687 Osterberg Jan. 2, 1945 1,487,174 Marden et a1 Mar. 18, 1924 2,339,392 Garner Jan. 18, 1944 2,281,474 Cartwright et al. Apr. 28, 1942 OTHER REFERENCES Mellor, Treatise on Inorganic and Theoretical Chemistry, vol. 7, page 227, publ. 1927, Longmans, Green 8: Co., 5th Ave., New York.

Thorpe, Dictionary of Applied Chemistry, vol. 5, page 478 (1916).

Strong, Procedures in Experimental Physics, pub. by Prentice-Hall Inc. (1939), pages 186 and 187. 

